Many inks and particularly those used in thermal ink jet printing include a colorant and a liquid which is typically an aqueous liquid vehicle. Some thermal ink jet inks also include a low vapor pressure solvent. When a substrate or a sheet of paper is printed with ink jet ink, the ink is deposited on the substrate to form an image in the form of text and/or graphics. Once deposited, the liquid is removed from the ink and paper to fix the ink to the substrate. The amount of liquid to be removed, of course, varies with the amount of ink deposited on the substrate. If a sheet is covered with 10% printing, as in text only printing, the amount of liquid to be removed is quite small. If the sheet is, covered with 90% printing, however, as when a graphic image is printed, the amount of liquid to be removed is substantially more and can cause image defects and paper deformation if not removed very rapidly.
Liquid can be removed from the ink and printed substrate by a number of methods. One simple method is natural air drying in which the liquid component of the ink deposited on the substrate is allowed to evaporate without mechanical assistance resulting in natural drying. Another method is to send the printed substrate through a dryer to evaporate the liquid. In some cases a special paper is used in which the liquid is absorbed by a thin coating of natural material deposited on the surface of the paper. Blotting of the printed substrate is also known.
In the case of natural drying, almost 100 percent of the liquid is absorbed into the paper and is then, over a long period of time, evaporated naturally. The absorption and desorption of water into and out of the paper, however, has some undesirable side effects, such as long drying time, strike through, feathering at edges of the printed image, paper curl and paper cockle. In the case of paper cockle, the absorption and desorption of the water relaxes the internal stresses of the paper and results in deformations known as cockle. Cockle is also a function of the amount of liquid deposited per unit area. Less printing on a page has less potential to develop cockle due to the smaller amount of liquid. More printing on a page has more cockle potential due to a higher amount of liquid per unit area. Cockle can also be induced by heating of the paper, which results in stress relief.
Ink compositions also have an effect on the drying rates and drying efficiency. For example, highly absorptive (fast drying) inks while requiring less ink to be removed by a dryer are prone to image quality defects such as feathering, raggedness, and strike through. On the other hand, slightly absorptive inks require more power from a dryer to dry since more ink requires evaporation.
The rate at which the image is dried is also critical for controlling the print quality. A slow drying rate can achieve ink permanence or drying effectiveness but also can result in image quality defects such as excessive image feathering or strike through. Additionally, a slow drying rate can result in image offset (ink from one sheet of paper is transferred to another sheet of paper because the ink has not dried completely), smear and spreading from contact with exit rolls, baffles and output stacking of the individual sheets. A very fast drying rate can result in image mottle and image spatter.
Drying rates are particularly critical when substrates are printed at high rates of speeds. Not only must image deformations and paper deformations be controlled, but the drying times must be short due to the high printing rates to ensure no offset at exit rolls.
A dryer must achieve image fixing (no offset/smear) and good image quality to reduce or prevent image disturbance, distortion, feathering and strike through. In addition the dryer must preferably reduce or eliminate cockle and curl. Besides the slow speed of conventional dryers, many dryers produce uneven drying rates resulting in uneven drying patterns. To shorten drying times, infrared drying techniques have been adopted. This method can, however, cause browning of paper during paper jams due to the elevated temperatures produced by the infrared heat.
Microwave dryers have been used for drying ink deposited on paper with varying degrees of success. Microwave dryers of various types are described in, for example, U.S. Pat. Nos. 3,584,389, 3,672,066, 3,739,130, 4,234,775 and 4,469,026. The efficient application of microwave power with a microwave dryer requires a properly tuned microwave circuit. Phase shifters are one way of tuning microwave circuits.
U.S. Pat. No. 3,617,953 to Kingma et al. describes a microwave impedance matching system for matching a microwave input waveguide to a microwave output waveguide. A first and second electromechanical phase shifter are moved transversely in waveguide sections to produce varying amount of differential phase shift.
U.S. Pat. No. 4,234,775 to Wolfberg et al. discloses a device which has a serpentine waveguide and uses microwave energy to remove moisture from a moving web. The microwave energy takes the form of standing waves which are purposefully disrupted to cause the peaks of the standing waves to continuously oscillate along the various sections of the waveguide, resulting in a more uniform application of the microwave energy across the width the web.
U.S. Pat. No. 4,286,135 to Green et al. describes a waveguide isolator having microwave ferrite bars to reduce energy reflected into the microwave source. A blower fan draws air past the microwave source and through a waveguide to provide cooling.
U.S. Pat. No. 4,332,091 to Bensussan et al. describes a microwave drying device intended for drying grains having a phase shifter allowing the phase to be adjusted to obtain maximum efficiency of the device.
U.S. Pat. No. 5,079,507 to Ishida et al. describes an automatic impedance adjusting apparatus for adjusting an impedance seen looking toward a microwave load. A cooling air outlet exhausts cooling air into a circular waveguide.